问如何获得点点的坐标？

EN

// Fetch offsetX & offsetY variables that contain the 
// canvas offset versus the window
// Re-fetch when the window is resized or scrolled
function reOffset(){
    var BB=canvas.getBoundingClientRect();
    offsetX=BB.left;
    offsetY=BB.top;        
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
window.onresize=function(e){ reOffset(); }

function handleMouseDown(e){

    // tell the browser we're handling this event
    e.preventDefault();
    e.stopPropagation();

    // calculate the mouse position
    mouseX=parseInt(e.clientX-offsetX);
    mouseY=parseInt(e.clientY-offsetY);

    // your stuff
}

function getMousePos(canvas, evt) {
  var rect = canvas.getBoundingClientRect();
  return {
    x: evt.clientX,
    y: evt.clientY
  };
}